Solar concentration systems typically entail optics (mirrors or lenses) to focus a large area of sunlight, or solar thermal energy, onto a small area. The solar thermal energy may drive a heat engine, such as a steam turbine, which may be further coupled to an electrical power generator to convert a portion of the solar thermal energy into electricity. Solar concentration systems may also drive a thermochemical reaction to generate a fuel that chemically stores a portion of the solar thermal energy. Water splitting, gasification of coal, and reforming of methane are all under investigation as potential solar thermochemical fuel production techniques. Solar concentration systems may drive other important reactions on an industrial scale as well, such as CO2 reduction into CO, for example.
Many solar thermochemical reactions entail a redox cycle. In a water splitting reaction to produce hydrogen from water, a metal-oxide redox pair is thermally reduced and the reduced reactive media then drives decomposition of water. The metal oxide is then reduced again to repeat the cycle. While identifying advantageous metal-oxides is currently a subject of research, thermodynamic considerations dictate the thermal reduction portion of the cycle generally requires a high temperature, typically between 1000-2000° C., depending on the reactive oxide chosen and other conditions in the system.
Solar thermochemical reactors can take many forms, affording more or less efficient fuel production, scalability, etc. One conventional system utilizes a honeycomb substrate that is coated with the reactive oxide. The honeycomb substrate is alternately exposed to collected solar energy to heat the system and reduce the reactive oxide, and to a reactant gas, such as H2O in the case of water splitting, to generate fuel. Such a reactor is essentially a fixed bed, and as such, suffers temperature non-uniformities and low thermal efficiency because much of the solar energy is expended on heating non-reactive portions of the bed (e.g., honeycomb substrate) and is ultimately rejected from the system as waste heat, rather than utilized for fuel production. Also, with each redox cycle, the entire system undergoes extreme thermal cycling, leading to component fatigue.
Additionally, other thermal swing process systems and methods may benefit from more cost efficient and productive systems and methods for heating the process material.
A system which avoids many of the difficulties and efficiency limitations associated with existing reactors would advantageously advance the art of thermal swing processing, and in particular, solar thermochemical fuel production.